# Fermilab

## Inquiring Minds

All About Light Main Page  |  Classical  |  Relativistic  |  Quantum

 Relativistic - Speed of Light and Einstein c and Einstein  |  c Top Speed  |  Speed & Speed  |  Faster than c? Mass Gain  |  E=m*c^2  |  Age Slower  |  Time Dilation  |  Twin Paradox Dopper Shift  |  Ticketed  |  Simultaneity  |  Competition  |  Fast Strudel Paradox? No!  |  Confusions  |
As we saw in previous pages, no electromagnetic wave propagates faster than the speed of light in vacuum. Einstein took this fact so seriously (of course, he was influenced by many other experimental results, such as the famous Michelson-Morley experiment) that he POSTULATED I repeat, he postulated the following:

• Postulate #1: The speed of light in a vacuum is the highest possible speed by which information can be exchanged between two events.

Please notice the careful wording, "highest possible speed, by which information can be exchanged." This statement is very strong, and has many deep consequences, as we will see later.

(Certain situations usually confuse people. One of those is the following: If you take two rigid rods that cross at an angle alpha, and one moves with respect to the other one with the speed v (say, perpendicular to the stationary rod), then you can always choose alpha small enough that the intersection point will move with the speed arbitrarily high. The solution to this "paradox" is that the notion of the rigid body does not make sense in special relativity. If you move one part of the rod, the other parts do not know they have to move until a signal arrives saying, "Hey, you should move." And this signal travels with a speed less than or equal to the speed of light.)

Furthermore, when Einstein built his special theory of relativity, he adopted one more postulate, something he remembered from his classical physics textbooks, namely that

• Postulate #2: ANY two inertial systems of reference (systems moving with respect to each other with a constant speed (not excluding a speed of zero) are equivalent. In other words, all the physical laws must have the same form in every inertial system.

After anchoring these two postulates, Einstein worked out all their consequences, and created the extremely successful special theory of relativity.

[At this point, it is appropriate to note that Einstein later (in 1921) went even further. He exchanged the word INERTIAL in the second postulate for the word ANY. After doing that, he built his famous GENERAL theory of relativity (the theory of gravity). I will not discuss this issue in these pages, but may in the future, if there is a demand for it.]

In the next couple of pages, I will address some of the most interesting consequences of Einstein's theory of special relativity.

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